Chris asked for police advice, Adam - are you speaking on behalf of the police?
Although I'm probably no more qualified to speak on behalf of the boys in blue than Adam, I'll risk seeing if you're happy in having it confirmed second-hand again Tim.
Having worked with several Police Wildlife Liaison Officers in the past, they'll always advise you not to challenge anyone, yet alone those apparently shooting illegally. Even down to doddery-looking egg thieves, just in case they lamp you one with their walking stick (which they usually only carry for grass parting and skull parting in an emergency).
If your local constabulary has a good wildlife Dept., make sure you carry a direct contact number, and use that if you want an immediate response on a wildlife crime. (After all, they have targets to hit just like the rest of the Force). If you have to ring 'normal' plod in an emergency you might find, as I have done a couple of times, you might find they have absolutely no knowledge of their specialist Dept and certainly won't divert bodies to a minor criminal offence, unless there is an assault and/or a weapon involved. (That was Kent, yours might be better.)
The best course is to collect evidence. O.K. they might get away with what they're doing, but often to get the person prosecuted fully the crime does actually have to take place so you have to grit your teeth about that. I've had to put up with things like watching barn owl chicks taken, hares coursed and badgers dug out over the years, and only some of them were ever done. But when it does work, it's worth it.
